I have not thought about this, but rsync works over FTP if you mount the FTP first.
Mount the FTP site
sudo curlftpfs -o umask=0777,uid=1000,gid=1000,allow_other ftp://user:pass@ftp.slusk.nu /media/slusk
rsync!
sudo rsync -av --delete /media/slusk/ /media/backups/slusk/
OK! it’s really really slow, but it works! :)